This invention relates to the field of single-operation cams and particularly to a cam mechanism in which a cam may be rotated indefinitely with follower means held in a waiting position until it is moved out of that position and forced, as by a spring, to travel to another point on the cam and into engagement there with a cam surface that will draw it back to the waiting position. In particular, the invention relates to an underbed thread trimmer for a sewing machine that incorporates a single-operation cam to drive hooks into position to engage the needle thread and looptaker thread at separate locations and to draw them back against ledger blades to sever both threads separately.
From time to time during a sewing operation, the operator cuts the needle and looptaker threads to withdraw the material from the sewing machine for any one of a number of purposes, such as to start a new seam or to begin work on new material. In order to withdraw material from the sewing machine, it is necessary for the needle to be in an elevated position, clear of the material being worked on. As a result, the needle thread is usually cut on the upwardly facing surface of the material being sewn. The looptaker thread engages the material from below, and the material is commonly turned over or withdrawn sufficiently from the stitch-forming region to allow access to the looptaker thread so that it can be cut on the opposite side of the material from the remaining end of the needle thread.
In order to keep the final stitches tight, it is common practice to backstitch before severing the threads. Alternatively, or in addition, after the threads have been severed one of them can be drawn through the material so as to be on the same side as the other thread, and the two ends can then be tied together to prevent any of the stitches from loosening.
Severing the threads by hand is undesirable in industrial sewing in which speed is frequently the most important consideration. As a result, industrial sewing machines frequently include a built-in trimming device that can be actuated whenever the operator desires to sever the threads. The most convenient locations for such severing, or trimming, devices is beneath the bed of the machine to sever the threads between the bed and the looptaker. Such devices have come to be known as underbed thread trimmers.
In industrial operations, the need for a high production rate makes manual tying of the severed threads too costly, and backstitching or the formation of a tack is used to hold the final stitches tight. Furthermore, it is desirable in industrial sewing to sever the threads as close to the material as possible to keep from wasting thread and to keep from having to trim the thread ends after they have been severed by the underbed mechanism. As a result, underbed thread trimmers on industrial sewing machines normally cut both threads at the same time and as close as possible to the material being worked on.
It is necessary to trim both the needle and loop-taker threads in such a way that there will be a sufficient length of thread to initiate the next stitching operation. The needle thread must not be trimmed so close to the needle or with the needle at such a location in its reciprocating cycle that the thread will be pulled out of the eye of the needle as the needle continues its cyclic motion.
A further problem associated with the provision of underbed thread trimmers is that of finding enough space for the mechanism. In the case of cyclic industrial sewing machines designed from the beginning with the recognition that underbed thread trimming mechanism will have to be incorporated, space to accommodate the mechanism can be reserved from the very beginning. Consumer sewing machines to be operated in the home are not usually as specialized as industrial machines and are likely to include a lot of mechanism that is not used in every sewing operation. Thus, an underbed thread trimmer for a consumer sewing machine must compete for space within the housing of the machine with other stitch-forming apparatus.
In my prior thread trimming mechanism shown and claimed in U.S. Pat. No. 3,386,402, the looptaker was a rotary hook with a bobbin in it and the main part of the trimming mechanism was separated from the stitch forming mechanism. In addition, another hook was arranged to grasp both the needle and bobbin threads at the same time and in the same location. Other underbed thread trimming devices are described in the following U.S. Pat. Nos.: 3,173,393; 3,359,933; 3,602,170; 3,605,664; 3,709,176; 3,776,161 and 3,921,554. None of the foregoing patents has a single-operation cam, as that term is used herein, and each of the patents provides means to bring both the needle thread and the looptaker thread into one place to be severed simultaneously by the cutting edge of a blade, either alone or acting in a scissoring action with a second blade at the same location.